gccint: Caller Saves

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1 18.9.10 Caller-Saves Register Allocation
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1 If you enable it, GCC can save registers around function calls.  This
1 makes it possible to use call-clobbered registers to hold variables that
1 must live across calls.
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1  -- Macro: HARD_REGNO_CALLER_SAVE_MODE (REGNO, NREGS)
1      A C expression specifying which mode is required for saving NREGS
1      of a pseudo-register in call-clobbered hard register REGNO.  If
1      REGNO is unsuitable for caller save, 'VOIDmode' should be returned.
1      For most machines this macro need not be defined since GCC will
1      select the smallest suitable mode.
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